[Cast] Increase number of encode threads to CEIL(N/2).
[Cast] Increase number of encode threads to CEIL(N/2).

For systems with 4 or fewer cores, the number of encode threads is
unchanged.  However, for higher-end systems, it makes sense to allow
more cores to be utilized to reduce frame encoding latency, especially
for key frames.

BUG=156767

Review URL: https://codereview.chromium.org/1152093004

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#331866}

Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Pluto-tv/chromium-crosswalk,Just-D/chromium-1,TheTypoMaster/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,Just-D/chromium-1,Chilledheart/chromium,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,Just-D/chromium-1,chuan9/chromium-crosswalk,Chilledheart/chromium,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Just-D/chromium-1,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk,axinging/chromium-crosswalk,Chilledheart/chromium,Chilledheart/chromium,Just-D/chromium-1,chuan9/chromium-crosswalk,hgl888/chromium-crosswalk,axinging/chromium-crosswalk,chuan9/chromium-crosswalk,Chilledheart/chromium,axinging/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,Just-D/chromium-1,axinging/chromium-crosswalk,axinging/chromium-crosswalk,hgl888/chromium-crosswalk,Pluto-tv/chromium-crosswalk,hgl888/chromium-crosswalk,chuan9/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,TheTypoMaster/chromium-crosswalk,Pluto-tv/chromium-crosswalk